Change FCurve handles from Python
You will find it here:

action=bpy.data.actions['yourAction']
#or
action=bpy.data.objects['yourObject'].animation_data.action
#
action.fcurves[fcurve_index].keyframe_points[kf_index].left_handle
action.fcurves[fcurve_index].keyframe_points[kf_index].right_handle


You can easily explore blender's data structures with the outliner using the datablocks option and the interactive python console using autocompletion (Ctrl+Space).

If you hover over properties in the editor with python tooltips enabled it often will show you the path to the property.

Hint:

If the object has not been animated yet and thus animation_data is None, use the object's method animation_data_create to create it.  

Afterwards assign an existing action to animation_data.action or create a new one:

action=bpy.data.actions.new('Your Name')


Note to the developers of blender:

In this case the path shown in the graph editor is abbreviated bpy.data.objects...left_handle and it is not possible to right click -> copy data path. So the hint shown is kind of useless.